1. Field of the Invention
This invention relates to an image forming apparatus having a plurality of image forming means based on different image forming methods.
2. Description of the Related Art
Image forming apparatuses such as copying machines and printers are frequently used in offices and the like, and apparatuses of this kind improved in image quality and recording speed have been developed. Recently, with the introduction of color techniques, there has been an increased demand for multicolor copying machines, multicolor printers and the like. However, the cost of multicolor recording is higher than that of monochromatic recording, and the recording speed of a multicolor recording system is considerably slower.
Multicolor copying machines and printers using a combination of different kinds of image forming means have recently been proposed. The structure of such apparatuses is, for example, as shown in FIG. 17. That is, image forming means A in accordance with an electrophotography method and image forming means B in accordance with an ink jet recording method are combined. As image forming means A, a photosensitive drum 50, a charging device 51, a development device 52, a transfer device 53, and a thermal fixation device 55 and other components are provided. The charging device 51, the development device 52 and the transfer device 53 are disposed around the photosensitive drum 50. Toner transferred to a recording medium 54 is fixed by the thermal fixation device 55. As image forming means B, a plurality of ink jet heads 56 are provided to perform multicolor recording. Recording medium 54 is transported from the image forming means A to the image forming means B.
In the above-described apparatus, in the case of black-and-white recording, the image forming means A produces an image at high speed with high image qualities, using the electrophotography method. On the other hand, in the case of multicolor recording, the image forming means B produces a multicolor image at low cost, using the ink jet recording method. It is thus possible to provide an apparatus capable of performing low-cost multicolor recording and high-quality, high-speed black-and-white recording.
In the above-described arrangement, however, a unit 57 for supplying recording medium to an image forming section is provided only for the image forming means A, and there are, therefore, the problems described below.
For example, in a case where recording medium 54 on which a document or a graphic has already been recorded by a different copying machine or the like is supplied to record characters "Secret Document" or the like in one color, e.g., vermilion, the recording medium 54 passes through the thermal fixation device 55 in the image forming means A before being transported to the image forming means B.
In such a situation, depending upon variations of the kind of recording medium 54, i.e., plain paper and the like (including, for example, surface properties and mold release properties) and variations in toner properties (e.g., variations in toner melting temperature), there is a possibility of the toner image already formed on the recording medium 54 being again melted when the medium passes through the fixation device 55. The molten toner attaches to a fixation roller 55a, and the toner on the roller again attaches to the transported recording medium 54. That is, there is a risk of occurrence of a phenomenon called "offset".
Also, an over head projector (OHP) sheet specially prepared for ink jet recording by being coated with a material for retaining ink thereon may be used as a recording medium. Such an OHP sheet cannot be passed through the fixation device 55, because the ink retaining material is easily affected by heat.
The order of image forming processing in the above-described conventional apparatus may be reversed, that is, the image forming apparatus may alternatively be such that image forming means A is provided as an ink jet system while image forming B is provided as an electrophotography system. Also in this case, the recording medium must pass through the fixation device 55, and the same problem is therefore encountered.
Also, even if an operator wishes to output only a color image (use only the ink jet system), it is necessary to operate the electrophotography system in order to transport recording medium 54. Unnecessarily large electric power is therefore required.
That is, if two or more kinds of image forming means are combined, a strong point of one of them, which is to be fully utilized, can be damaged due to a weak point of another one of them.